Fix rubocop errors

Author: jez

lib/jekyll-feed/generator.rb

@@ -69,16 +69,7 @@ def collections
 
       @collections = normalize_posts_meta(@collections)
       @collections.each_value do |meta|
-        meta_categories = meta["categories"]
-        for_collection = case meta_categories
-                         when Array
-                           meta_categories
-                         when true
-                           @site.categories.keys
-                         else
-                           []
-                         end
-        meta["categories"] = for_collection.to_set
+        normalize_categories(meta)
       end
 
       @collections
@@ -151,6 +142,19 @@ def normalize_posts_meta(hash)
       hash
     end
 
+    def normalize_categories(meta)
+      meta_categories = meta["categories"]
+      for_collection = case meta_categories
+                       when Array
+                         meta_categories
+                       when true
+                         @site.categories.keys
+                       else
+                         []
+                       end
+      meta["categories"] = for_collection.to_set
+    end
+
     def disabled_in_development?
       config && config["disable_in_development"] && Jekyll.env == "development"
     end